a) Field of the Invention
This invention relates to optical head apparatus used in recording, regeneration for optical recording medium such as DVD (digital versatile disk).
b) Description of the Related Art
In optical head apparatus used in recording, regeneration for optical recording disk such as CD (compact disk) and DVD, in general, a laser beam-emitting element, a prism for the incident laser beam emitted from this laser beam-emitting element, a collimating lens to convert laser beam transmitted through this prism to parallel light flux, and an objective lens to condense parallel light flux emitted from collimating lens on optical recording medium, are located on an optical path in this order.
Here, the prism is used as an optical path synthesis element or optical path separation element. For example, in an optical system shown in FIG. 1, it is used as an optical path separation element. Here, prism 4 is provided with the No. 1 prism structural member 41 and the No. 2 prism structural member 42 joined through partially reflecting face 43 slanted toward the optical axis of a laser beam emitted from laser beam-emitting element 2. Partially reflecting face 43 transmits a laser beam emitted from laser beam-emitting element 2 toward collimating lens 5, and on the other hand, reflects light returning from optical recording medium 20 toward light-receiving element 9.
In optical head apparatus constructed in this fashion, a laser beam emitted from laser beam-emitting element 2 is transmitted through prism 4 as divergent light, and enters collimating lens 5. Consequently, unless there is sufficient control of astigmatism, small spots cannot be formed on optical recording medium 20. Therefore, for prism 4, identical materials are used to construct the No. 1 prism structural member 41 and the No. 2 prism structural member 42, and at the same time, surface precision and extent of parallelism of the No. 1 prism structural member 41 and the No. 2 prism structural member 42 are subjected to control measures.